It was noticed that not all accounts exist on the installation - e.g. there is 1000001, 1000002, 1000004, but no 1000003. Why?
This is correct in most cases. The absence of some account ID means that the transaction to create this account has been rolled back - e.g. the API call was rolled back, an order was not placed because of failed fraud screening performed before order placement or some system exception occurred.
New account will be created with the next id from the sequence.